# ohmmeter

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## ohmmeter

(ōm`mē'tər), instrument used to measure the electrical resistanceresistance,
property of an electric conductor by which it opposes a flow of electricity and dissipates electrical energy away from the circuit, usually as heat. Optimum resistance is provided by a conductor that is long, small in cross section, and of a material that conducts
of a conductor. It is usually included in a single package with a voltmetervoltmeter,
instrument used to measure differences of electric potential, commonly called voltage, in volts or units that are multiples or fractions of volts. A voltmeter is usually combined with an ammeter and an ohmmeter in a multipurpose instrument.
, and often an ammeterammeter
, instrument used to measure the magnitude of an electric current of several amperes or more. An ammeter is usually combined with a voltmeter and an ohmmeter in a multipurpose instrument.
. In normal usage, the ohmmeter operates by using the voltmeter to measure a voltage drop, then converting this reading into a corresponding resistance reading through Ohm's lawOhm's law
[for G. S. Ohm], law stating that the electric current i flowing through a given resistance r is equal to the applied voltage v divided by the resistance, or i=v/r.
. If the current is known, the voltage drop across the unknown resistance may be read to give the resistance directly. If the current is not known, or if it is not possible to measure the voltage across the unknown directly, reading the voltage drop across a known resistance in the same circuit will give the current, and once this voltage is subtracted from the total drop for the circuit, the voltage drop across the unknown, and thus its resistance, may be found.

## Ohmmeter

A portable instrument for measuring relatively low values of electrical resistance. The range of resistance measured is typically from 0.1 microhm to 1999 ohms (&OHgr;). The ohmmeter solves quickly and easily a variety of measurement problems, including measuring the resistance of cladding and tracks on printed circuit boards, electrical connectors, and switch and relay contacts, as well as determining the quality of ground-conductor continuity and bonding, cables, bus-bar joints, and welded connector tags. See Resistance measurement

## Ohmmeter

a direct-reading instrument for measuring electric true (ohmic) resistances. Among the types of ohmmeter are megohmmeters, teraohmmeters, and microhmmeters, which differ in the ranges of the resistances being measured. Ohmmeters are made with a permanent-magnet measuring head or a permanent-magnet quotient meter.

The operation of a permanent-magnet ohmmeter is based on measurement of the current flowing through the resistance being measured, when the voltage from the source is constant. To measure resistances in the range from several hundred ohms (Ω) to several megohms (MΩ), the meter and the resistance rx being measured are connected in series. In this case, the current I flowing through the meter and the deflection α of the moving part of the instrument are proportional: I = Cα = U/ro + rx); α = U/C(r0 + rx), where U is the voltage supplied by the source and r0 is the resistance of the meter. For small values of rx (up to several ohms), the meter and rx are connected in parallel. If U and C are constant, the deflection α depends on rx; therefore, the scale of the meter can be calibrated in ohms for easier readout. The error of such an ohmmeter is 5–10 percent of the length of the scale used.

An ohmmeter is frequently part of a multimeter, such as the VOM (volt-ohm-milliammeter). The bridge method of measurement is used in ohmmeters for work requiring great precision. Electronic amplifiers are used to increase the sensitivity of the meter and to improve measurement accuracy.

Electronic ohmmeters with digital readout of the resistance being measured were introduced in the 1960’s. Some ohmmeters can be connected to computers. The range of measurement of such ohmmeters is 1 mΩ to 100 MΩ or more; the error is 0.01–0.05 percent.

### REFERENCES

Shkurin, G. P. Spravochnik po elektro- i elektronnoizmeritel’nym priboram. Moscow, 1972.
Spravochnik po elektroizmeritel’nym priboram. Edited by K. K. Iliunin. Leningrad, 1973.

E. G. BILYK

## ohmmeter

[′ō‚mēd·ər]
(engineering)
An instrument for measuring electric resistance; scale may be graduated in ohms or megohms.

## Ohmmeter

A portable instrument for measuring relatively low values of electrical resistance. The range of resistance measured is typically from 0.1 microhm to 1999 ohms (&OHgr;).

The ohmmeter solves quickly and easily a variety of measurement problems, including measuring the resistance of cladding and tracks on printed circuit boards, electrical connectors, and switch and relay contacts, as well as determining the quality of ground-conductor continuity and bonding, cables, bus-bar joints, and welded connector tags. See Resistance measurement

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